Adipose tissue macrophages (ATMs) regulated adipose tissue inflammation is an important contributor to the pathogenesis of insulin resistance associated disorders, including type 2 diabetes and cardiovascular diseases. Chronic nutrient excess induces ATMs to undergo a distinct phenotypic switch (macrophage polarization) from anti-inflammation (M2) in lean tissues to pro-inflammation activation (M1) in obese tissues. We recently discovered, for the first time, that miR-223 is a potent regulator of diet-induced adipose tissue inflammation, which primarily acts through controlling macrophage polarization. Moreover, miR-223 is dramatically decreased in human diabetes patients, reinforcing the potential function of miR-223 in adipose tissue inflammation. However, to date, it remains unknown how miR-223 exactly regulates macrophage polarization and promote these ATMs toward M1 activation under challenges. Our preliminary studies showed that miR- 223 is markedly increased during M2 activation which can be further enhanced by a PPARgamma agonist; and loss of miR-223 in macrophages results in blunted M2 activation to PPARgamma agonist. Moreover, genomic survey predicted three PPARgamma consensus binding elements upstream of miR-223 precursor. Thus, we hypothesize that miR-223 is a potent macrophage M2 activator and a PPARgamma-miR-223 circuit is an important pathway to control ATM polarization in chronic nutrient excess induced adipose tissue inflammation and systemic insulin resistance. Three specific aims will be carried out to address this hypothesis: Specific Aim 1: Determine the mechanisms of PPARgamma-miR-223 circuit in regulating macrophage polarization. We will employ macrophages with altered miR-223 levels and specific PPARgamma agonist or inhibitor to dissect in detail the functional interactions between PPARgamma and miR-223 during macrophage activation. Specific Aim 2: Determine the role of miR-223 in PPARgamma action against adipose tissue inflammation and insulin resistance. We will use gain- and loss-of miR-223 mouse models with PPARgamma agonists and antagonists to determine the role of miR-223 in protective effects of PPARgamma against excess nutrients induced adipose tissue inflammation and systemic insulin resistance. Specific Aim 3: Identify the target genes of miR-223 that are required for the PPARgamma-miR-223 pathway. To dissect the PPARgamma-miR-223 pathway, we will identify miR-223 target genes and validate their functions in regulating macrophage polarization and adipose tissue inflammation. The overall goal of this project is to determine the molecular mechanism underlying miR-223 mediated regulation of macrophage polarization and adipose tissue inflammation. Completion of this project will provide a novel microRNA-based paradigm with respect to ATM polarization. Moreover, this study will provide crucial insights for developing new therapeutic strategies in mitigating adipose tissue inflammation associated diseases using miR-223 mimetics or antagonists of miR-223 regulated target genes.